In most spontaneously ovulating mammals the corpora lutea formed at ovulation secrete progresterone for 2 weeks. The corpora lutea formed at ovulation in the rat secrete progesterone for only a few hours during the estrous cycle. However, a prolonged luteal phase or pseudopregnancy (psp) of approximately 2 weeks can be induced in the rat by mechanical or electrical stimulation of the uterine cervix (CS) on proestrus. Such stimulation leads to 10 days of daily nocturnal and diurnal surges of prolactin (Prl) secretion from the pituitary gland. This hormone is responsible for maintenance of the corpus luteum. The CS changes the activity of 2 areas of the hypothalamus to initiate these surges. It turns off inhibitory neurons for nocturnal surges and excites stimulatory neurons for diurnal surges within the preoptic area. Moreover, the stimulus excites stimulatory neurons for both surges within the dorsomedial-ventromedial areas of the hypothalamus. The initiation of the surge is not ovarian-dependent. Attenuated nocturnal and diurnal surges of Prl can be initiated and persist in CS-OVX rats. Progesterone will enhance the nocturnal surge and prolong the incidence of both surges. Estradiol will magnify the diurnal surge and abolish the nocturnal surge. The termination of psp is due to the waning secretion of progesterone and the heightened secretion of estradiol leading to extinction of both Prl surges. The uterus also contributes by acting directly at the hypothalamo-pituitary axis to terminate Prl surges. Utilizing radioimmunologic estimates of hormone levels in the blood as well as appropriate hypothalamic lesions and stimulation, the functional interrelationship of ovaries, uterus and hypothalamus required for initiation of Prl surges by CS will be determined. In addition, the sexual specificity of hypothalamic areas controlling this unique pattern of Prl secretion will be identified.